Hi ​@Brierleys ,

Could you please try the below steps for me.

Go to your phone Settings
Tap Phone
Tap My Number
Enter the correct number using +44 instead of the first 0

Then:

Go back to Settings
Tap Messages
Make sure iMessage is turned ON
Tap Send & Receive
Check that the correct number is selected

Alternatively, you can also try the following:

Go to Settings and tap your name
Tap Sign-In & Security
Tap your primary phone number to make changes
To remove the number from your account, tap Remove from Account
Then choose Add New Primary Phone or Email

Please let us know if this resolves the issue for you.

-Michael Z